1. Field of the Invention
The present invention relates to a squelch circuit for a transmitter/receiver, especially an FM radio transmitter/receiver, and, more particularly, to a squelch circuit that outputs a signal for a predetermined length of time, although the squelch signal is set in a "mute" state, thereby allowing an operator to adjust the volume while listening to sounds produced from a loudspeaker.
2. Background Art
A conventional squelch circuit is described below with reference to FIG. 2. In this squelch circuit, as shown in FIG. 2, received audio signal 11 is input to switching unit 15 and enabled/disabled by squelch signal 12. Received audio signal 11 is disconnected when squelch signal 12 is set in a "mute" state, and the received audio signal 11 is output to amplifier 14 when squelch signal 12 is set in a "non-mute" state. Received audio signal 11 is amplified by amplifier 14 to a volume level that is adjustable via variable resistor 16. Received audio signal 11 is then output to loudspeaker 13.
In a conventional squelch circuit, although an operator can adjust the volume controller (consisting of variable resistor 16) while listening to sounds from loudspeaker 13 in a "non-mute" state, i.e., a radio wave receiving state, the operator cannot adjust the volume controller while listening to sounds in a "mute" state. For this reason, when the volume controller is adjusted while in a "mute" state, the volume controller is roughly adjusted according to the scale of variable resistor 16. In another method, a squelch volume control in the squelch circuit is manually set in a "non-mute" state temporarily, so that the volume is controlled by variable resistor 16. Then, the squelch volume control is adjusted to be set in a "mute" state again. This last method is inconvenient for the user.
Volume adjustments in a mute state must be frequently performed when using a radio transmitter/receiver. For example, immediately after the radio transmitter/receiver is turned on, it has an unknown volume value. Also, when the variable resistor 16 is changed while in a "mute" state, the adjustment position in a "non-mute" state becomes unknown.